Microscopic models for time-resolved photoluminescence in -PTCDA single crystals R Scholz, A Yu Kobitski, I Vragovi´ c, T U Kampen, D R T Zahn Institut f ¨ ur Physik, Technische Universit¨ at Chemnitz, Germany Wagner H P Department of Physics, University of Cincinnati, USA Abstract. In the present work, we analyse the radiative and non-radiative recombination channels in -PTCDA (3,4,9,10 perylene tetracarboxylic dianhydride) by applying time- resolved photoluminescence (PL) techniques in the 100 ns range between temperatures of K and K. These experimental findings are interpreted with microscopic calculations of the dispersion of Frenkel excitons and the investigation of different dimer geometries using time-dependent density functional theory. Based on the latter approach, we can identify radiative recombination from excimer states and from anion-cation pairs. From the temperature dependence of the different PL channels we gain some insight into radiative lifetimes and the energy barriers for non-radiative de-excitation. 1. Introduction Among the molecular semiconductors, some substances like Alq show a remarkably high photoluminescence (PL) efficiency and are therefore well suited for organic light emitting diodes, while many layered crystalline materials like thiophenes or perylene derivatives are poor emitters [1, 2]. The PL efficiency can be increased by adding functional groups reducing the aromaticity of the molecules [3, 4]. For both classes of organic substances, such functionalizations result in deviations from the planarity of the molecules and in a larger distance between different molecular sites. As a consequence, the inter-molecular interactions are reduced, and the corresponding PL quenching mechanisms become less efficient. On the other hand, crystals of planar molecules are interesting model systems for studying the influence of the inter-molecular interactions on the PL spectra. In the case of crystalline PTCDA, the stacking distance is much shorter than in the crystalline phases of other perylene derivatives, so that this substance is particularly suited for such investigations. Previous time-resolved PL studies of PTCDA and MePTCDI samples have revealed PL decay times times of about 4 ns in the case of dissolved monomers and amorphous samples [5, 6], while poly-crystalline PTCDA films of sufficient thickness show a radiative lifetime of about 11 ns at low temperatures [7, 8]. In the present work, we extend these investigations to the temperature dependence of time-resolved PL measured on single crystals of -PTCDA [9, 10]. Our results are interpreted with microscopic calculations based on the transfer of Frenkel excitons [11, 12] and with the investigation of different dimer geometries using time-dependent density functional theory (TD-DFT) [13], together with models for activated non-radiative decay.